1. Field of the Invention
The present invention relates to a process for the gas-phase hydrogenation of free fatty acids, optionally in admixture with fatty acid esters, to fatty alcohols on particulate fixed-bed metal oxide catalysts.
2. Description of the Prior Art
Fatty alcohols, i.e. predominantly linear monohydric alcohols having chain lengths of 8 and more carbon atoms, and their production are described in detail in the literature, for example in Ullmanns Enzyklopaedie der technischen Chemie, 4th Edition, Vol. 11, pages 427-445. A preferred starting material for their production are the fatty acids or fatty acid mixtures occurring in natural fats and/or oils, which are converted by catalytic hydrogenating reduction into fatty alcohols of corresponding chain length. On an industrial scale, the hydrogenating reduction is normally carried out on fatty acid methyl esters obtained from the natural starting materials by transesterification or by lipolysis and subsequent esterification of the free fatty acid with methanol. The use of the fatty acids to be reduced in the form of their methyl esters protects the catalysts, particularly against aggressive attack by the free carboxyl group, so that on an industrial scale satisfactory volume-time yields can be obtained over sufficiently long periods. Accordingly, native fatty alcohols are now predominantly obtained from fatty acid methyl esters by the gas-phase hydrogenation process in which the distilled methyl esters are passed in the vapor phase over fixed copper-containing mixed-oxide catalysts, together with a large excess of hydrogen, at temperatures of from about 230.degree. to 250.degree. C. and under pressures of from bout 250 to 300 bars. The copper-containing mixed-oxide catalysts produced by co-precipitation using the wet method are used in the form of particulate catalysts or extrudates and are generally reduced in the the direct hydrogenating reduction of the free fatty acids has hitherto been of no real practical significance.
It is known that hydrogenation of free fatty acids to form fatty alcohols can be carried out by the suspension process using copper chromite as catalyst. However, this process is only of any use when the copper chromite catalyst is obtained by decomposition of the copper-ammonium chromate complex initially obtained, followed by washing with acetic acid. Catalyst produced in this way is particularly expensive and, effectively, can only be used for suspension hydrogenation because it is extremely difficult to convert into a fixed-bed catalyst. Acid-washed copper chromite can only be tableted with considerable difficulty, if at all, and because of this cannot be converted into abrasion-resistant, mechanically stable extrudates or other moldings. Any attempts to induce this solidification by tempering adversely affects the catalytic effect. Attempts to apply acid-washed gel or aluminium oxide, also failed to produce technically useful results. The support peptizes and the catalyst is readily washed off the support. In addition, supported catalysts have other disadvantages compared with solid catalysts.
Fatty acid esters, particularly fatty acid methyl esters, and free fatty acids are frequently mentioned together in the relevant patent literature as starting materials for the hydrogenating reduction to saturated and/or unsaturated fatty alcohols (cf. for example U.S. Pat. Nos. 3,193,586 and 3,173,959 and German application Nos. 2,513,377 and 2,613,226). However, these disclosures should be regarded entirely differently with respect to their practical implementation on an industrial scale, depending on whether the fatty acid methyl esters or the free fatty acids are used as starting material in the hydrogenation stage. The fact is that, hitherto, the considerable advantages of fixed-bed catalysis using solid catalysts have not been successfully developed for processing a starting material consisting of or containing free fatty acids. The corrosive effect of the free acids at elevated temperature and pressure upon the solid catalysts which, basically, have been successfully used in the reduction of methyl esters is so great that, hitherto, no serious consideration has been given to the commercial implementation of the above-mentioned proposals for the reduction of free acids.
In order to enable copper chromite catalyst which has not been washed free from acid, i.e. is more favorably priced, to be used for the hydrogenation of free fatty acids, the following procedure is adopted in practice: Fatty alcohol is introduced in a relatively large quantity into the hydrogenation reactor, after which free fatty acids are fed in under hydrogenation conditions. The small quantitites of fatty acid introduced are initially esterified with the excess of fatty alcohol present in the hydrogenation reactor. This process may be carried out continuously using the less expensive catalysts which have not been washed with acid. However, the process requires disproportionately large reactors and only reaches conversion levels of 96% whereas processes using fixed-bed catalysts achieve conversion levels of 99% and higher.